Photoinduced reactions of isatin and N-methyl-1,3,4-isoquinolinetrione with bicycloalkylidenes such as bicyclopropylidene, cyclopropylidenecyclobutane, cyclopropylidenecyclohexane and bicyclohexylidene were investigated. The reactions gave spirooxetanes as the major products derived from the [2 + 2] photocycloaddition pathway via 1,4-biradical recombination. Unusual products including the [4 + 2 + 2] cycloadducts, the oxoisochroman derivatives and other ring-rearranged products were derived from competitive pathways via 1,6-biradical recombination. The presence of oxygen in the reaction solution was found to be relevant to the distribution of different types of products. Mechanisms were proposed to rationalize the chemo- and regioselectivity in the photoreactions and the origin of the different types of products.